In some communication systems, rather than using a single signal, a plurality of signals may be used to transmit data. For example, Sirius XM Radio Inc., which merged from two originally separate and independent satellite broadcast companies, specifically, Sirius Satellite Radio Inc. and XM Satellite Radio Inc., utilizes a network of satellites and terrestrial repeaters to transmit data over a plurality of signals. Such a network may include six satellite signals and three terrestrial repeater signals, with signals originating from the Sirius side occupying a lower frequency range (e.g. 2320 MHz-2332.5 MHz), and signals originating from the XM side occupying a higher frequency range (e.g. 2332 MHz-2345 MHz).
In such a multi-signal system, frequency offsets often occur that need to be corrected either at the source of transmission and/or at the receiver. For example, the transmitter itself may not be transmitting at the precise frequency. In the case of a satellite signal, a system failure, signal drifts, and/or calibration errors at the satellite are some examples that may cause the satellite signal to be transmitted with a frequency offset at the source. In the event of a failure of a satellite frequency translator, the frequency may vary throughout the day, even moving constantly. Similar problems may also be present at a terrestrial repeater.
In the case of a mobile receiver, the frequency offset due to environmental influences (e.g., temperature fluctuations) is constantly changing as it travels and therefore needs to be corrected so that the transmitted signal may be processed and used by the mobile receiver.
The design of a receiver may be relatively straightforward if there is a separate frequency tuner for each signal. However, such an approach may increase receiver costs, footprint, and may result in an overall inefficient receiver design.
Additional factors at the receiver may also contribute to frequency offsets that require correction. For example, it may be desirable to use inexpensive oscillators in the frequency tuner to reduce cost, but such oscillators may be less accurate due to variations in their crystal oscillator frequencies. Plus, the oscillator frequency may vary significantly based on temperature. This is a particular challenge in the case of a car receiver, which may experience significant temperature swings (and corresponding frequency offsets), for example, from cold startup to a desirable operating temperature.
To correct frequency offsets of the different signals, traditional automatic frequency control (AFC) correction methods for multi-channel satellite radio systems generally measure the frequency offset from a single demodulator corresponding to a particular signal and then apply a correction by adjusting a center frequency of the tuner of the receiver. Although this may correct the frequency offset for that particular signal, if other signals have offsets that differ from that of the signal used to make the tuner correction, these other signals may require additional frequency corrections, which the traditional AFC would fail to correct. Additionally, this requires the tuner to be capable of having its center frequency externally controlled, which may add complexity, cost and may necessitate calibration of a switched capacitor network used for center frequency adjustments.
For example, in a multi-channel system, such as Sirius XM Radio Inc.'s system, if the receiver implementing traditional AFC method is using a satellite signal as the reference signal, the receiver would be constantly monitoring that satellite reference signal and measuring and correcting for the frequency offset with respect to that reference signal. If that reference signal is lost, however (for example when the receiver is travelling in a metropolitan area and that signal is blocked by a tall building), the receiver will need to switch to another signal, e.g., a terrestrial signal to try to pick up the broadcast. But because the frequency offset from the satellite signal may be different from the terrestrial signal, a reference frequency offset from the satellite signal may not correct the frequency offset from the terrestrial signal when the receiver makes the switch, or vice versa, resulting in loss of decoded signal.
The traditional correction method is particularly problematic in the case of a satellite signal and a terrestrial signal that differ in frequency by more than one frequency bin when using frequency division multiplexing (e.g., Coded Orthogonal Frequency-Division Multiplexing (COFDM). For example, if the satellite and terrestrial signals are alternately present, an amount of AFC correction may be determined based on a measured offset for whichever signal is currently present. The center frequency of the tuner may then be adjusted accordingly based on the amount of the AFC correction. If the frequency offset between the two signals is greater than a width of a COFDM frequency bin, the receiver may need to determine which COFDM frequency bin contains the corresponding data and switch to that frequency bin. Otherwise, data may be assigned to incorrect bins leading to forward error correction (FEC) block errors. This may result in brief muting of the decoded signal until the frequency offset is accurately determined and compensated for. In the case of satellite radio, this means that the listener would hear a silent gap in the audio program. Therefore, there exists a need to alleviate these problems by avoiding the need to make adjustments to the center frequency of the tuner. The design of a receiver provided with a tuner that may handle more than one signal may result in, for example, a lower cost, a smaller footprint, and/or increased efficiency of the receiver and a seamless user listening experience.